1. Field of the Invention
The present invention relates to a backlight module and a liquid crystal display (LCD) employing such a backlight module, and more particularly, to a backlight module which can detect abnormal lamp tubes and an LCD having such a backlight module.
2. Description of Prior Art
With a rapid development of monitor types, novel and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDAs), digital cameras, and projectors. The demand for the novelty and colorful monitors has increased tremendously.
Most of the TFT-LCDs utilize Cold Cathode Fluorescent Lamps (CCFL) as backlight sources. The CCFL can emit light when noble gas inside the lamp is driven by a high-frequency driving voltage. In addition, the required driving voltage increases as CCFL length increases. Yet, the current leakage also increases as CCFL length increases and an increase in the operating voltage. Therefore, the length of the CCFL and the operating voltage applied to the CCFL are associated with an increasing amount of current leakage.
Conventionally, a commonly-used method of enabling the CCFL is to apply a high frequency driving voltage on one end of the CCFL and the other end is coupled to ground or to be floated. Thus, the lamp tube has a high voltage on one end and a low voltage on the other end. The higher the operating voltage is, the non-linearly greater the current leakage is. Furthermore, a decrease in current is varied as an increase in distance far from the high voltage end of the CCFL, thereby incurring uneven display quality and uneven brightness contrast.
For that reason, traditional inverters usually employ a transformer with a primary winding in series to balance the current output to the lamp tube. But it requires a larger printed circuit board (PCB) and higher design costs to use a transformer to balance the current. So, another design method, that is, ballast capacitors coupled to the lamp tube, is employed in the inverter. This design method is characterized by simplicity and cheapness, so design costs can be reduced. This method mostly uses a single transformer to activate all of the lamp tubes and then utilizes ballast capacitors to be a balancing device of the current of the lamp tube. However, the greatest risk to this method is that, a protection circuit cannot perform its function when the lamp tube is malfunction or the ballast capacitor cracks/is poorly soldered; it contains some potential dangers.